A known technique for coating powder particles comprises the use of a fluid bed plant. By means of this type of equipment, the powder particles are fluidized in air, the coating composition is atomized over the fluidized powder particles, and the coated particles are dried by continued passage in the fluid bed plant.
This type of fluid bed treatment is suitable only when coating powder particles have a size which is several times larger than the droplets of the atomized coating composition. When the particles are only a few times larger than the droplet size or smaller, an agglomeration occurs which results in the particles gluing together in clusters with large interstices of air, which is normally an undesired effect in a coating process.
In another technique, which is especially adapted for coating of relatively heavy particles in a fluid bed plant, a spray nozzle is placed at the bottom of a vertical tube in the center of fluidized powder particles and atomizes the coating composition upwards through the tube and concurrent with the powder particles, which particles are carried upwards through the same tube by means of an air flow. When the powder particles are coated, they are carried upwards, are dried, fall down into the fluidized powder particles and into the tube, and are coated with a new layer of the coating composition. This process is continued until the desired thickness of the coating layer has been obtained.
One problem associated with these two methods is that it is normally only possible to apply a very thin layer of a coating composition in each application stage. If a thicker layer of coating substance is applied at one time, the powder particles will tend to agglomerate due to sticky surfaces.
One known technique for coating of particles is to disperse powder particles in a liquid coating composition and spray-drying this mixed dispersion. However, in many cases, it is not realistic or at least not optimal to use this technique, in particular where the dispersion of the particles in the liquid coating composition will increase the viscosity to such an extent that spray atomisation of the dispersion becomes difficult or even impossible.
U.S. Pat. No. 3,615,723 describes a spray-drying process wherein the droplets, which are still tacky, are discharged at a screen where a mat is formed and dried. At least portions of the mat are subdivided into a particulate form (fines) and recirculated to the spray-drying chamber. The recirculated fines are added through a pipe discharging in a certain distance from the atomization zone. The dried, but still tacky droplets are discharged at the said screen in order to obtain an agglomeration of the particles. No coating is carried out by this process, and the dust particles are often over-heated due to lack of evaporation from the surfaces.
In U.S. Pat. No. 3,919,096, it is attempted to solve this problem by dispersing the particles in a liquid which is neutral to the particles and which is thereupon spray dried from the central channel of a special nozzle with three channel systems. The coating agent is sprayed from the outer channel system and the compressed air for atomizing both components is added through the intermediate channel.
U.S. Pat. No. 3,615,723 discloses recirculation, in connection with spray drying, of the smallest particles (dust) of already dried material to the spray drying chamber. The addition is performed through a tube the outlet of which is remote from the nozzle. The purpose of this process is an agglomeration of the dust with partially dried drops to obtain a uniform agglomerate size in the end product.
DK published Patent Application No. 160809 discloses agglomeration of substances in connection with spray drying, the fines (the non-agglomerated particles) being recycled to the central part of the atomization zone. The fines are dispersed in a minor part of the drying air, and the position for introducing the fine particles is controlled in such a manner that the average distance from the spraying wheel to the region in which the fine particles encounter the atomized droplets or partially dried particles is controlled. It is mentioned that formation of unwanted coated particles may take place when having a very short distance between the atomization wheel and the place for addition of powder particles.
EP patent application publication No. 344 375 discloses a spray-drying method and equipment for concurrent particle coating. The method is an application of a liquid substance, e.g. lecithin or other non-oxidative substance to particles which are derived from a mixture of solid particles and liquid by means of the spray-drying equipment.
EP patent application no. 423 701 A2 describes a method for granulation and coating by means of an equipment which uses drying air streams to effectuate granulating, coating and drying. The method comprises a supplying of an article to be coated and a supplying of a coating material, which supplyings are provided in such positions that the article to be coated and the coating material collide with each other before the article to be coated and the coating material are dispersed by the drying air streams. Cold air is supplied into a region where the article to be coated and the coated material collide with each other and hot air is supplied to a region spaced farther apart from the nozzles for supplying the article to be coated and the coating material than the region of the cold air supply.